Reduced speed and its effect on discharge pressure

[hysim]

Hi all,

I'm actually trying to study the valve opening and closing motion of reciprocating compressor at different pressures.
I plan to connect 2 single stage air compressor in series, the first stage with 710 lpm while the second stage with 340 lpm.
I want to reduce the speed of my first stage compressor by using a VFD connected to the motor.
1) What will be the effect of speed reduction on the discharge pressure?
From my understanding, lower speed means the compressor is supplied with lower capacity, but since the suction pressure is still the same (since it draws air from atmosphere), the piston may need to move further downwards for the suction valve to open. Therefore the suction valve may open late and close earlier at lower speed. So smaller amount of mass enters the cylinder and therefore discharge pressure should be reduced? What about compression ratio? Is the compression ratio solely dependent on dimension of piston and power of compressor? I'm quite confused with this.

2) How should I know that whether the second stage compressor can sustain given the high suction pressure supplied from the first compressor? Anything to consider in selecting the second stage compressor?

Thank you.

 

[nickelkid]

So, you have 2 compressors in series, each with their own driver. How are you maintaining 2nd stage discharge pressure?

 

[LittleInch]

The key here is your thought "the piston may need to move further downwards for the suction valve to open.~" - Err why? If the piston is moving slower, my thought is that if anything it could open sooner.

When you slow the speed down, you slow the volume rate down, not pressure. If you slow it down / reduce power too much at some point the compressor will stall as you don't have enough torque to convert to axial piston force to compress the gas.

With two in series you need to match mass flow and speed carefully to maintain the required interstage pressure and outlet pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[nickelkid]

Inter stage pressure may be able vary depending on the mechanical ratings, but not much information to understand what hysim really is doing or has.

 

[zdas04]

The recip is positive displacement. That means that the piston sweeps the same volume every revolution regardless of speed. For an absolutely fixed suction pressure (not really possible), the valves will open in exactly the same position every time regardless of driver speed. I say that a constant suction pressure is not possible because there are always losses between the atmosphere as a whole and the top side of the suction valve (friction in pipes, resistance in mechanical filters, resistance in dryers, etc). All of these resistances are velocity dependent so as you speed the machine up, the actual pressure on the top side of the suction valves goes down, so the cylinder pressure required to open the suction valve goes down (i.e., the valve opens later at higher speeds).

I've done tests with very sensitive temperature instruments in the inlet plenum and in the outlet plenum and used the adiabatic heat of compression equation to find the pressure when the suction valve opened. I got a variation in crank angle of nearly 5 degrees between the slowest speed I tested and the fastest speed I tested.

On the discharge side, the valve will not open until it can (i.e., the pressure within the cylinder is slightly greater than the outlet plenum pressure plus the inherent valve stiffness. That pressure is determined by downstream pressure. Whatever downstream pressure you give it, it will match--until the downstream pressure gets so high that a rod breaks, a mechanical coupling fails, or you stall out the driver (never seen that one, but it is possible).

As to matching the two machines, that is usually done via capacity control on the second machine. This means that you put a suction control valve, a VFD, or an auto-unloader on the second machine that is looking at suction pressure (preferably in the inlet plenum, but usually at the skid edge) and adjusting the compressor to keep suction pressure constant.

Your second question does not make sense to me.


David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[hysim]

Hi nickelkid,
That's part of my problem too. Perhaps to put a suction control valve before the air enters the second compressor, as suggested by David.

 

[hysim]

Hi LittleInch,

My understanding is that the valve is open or close due to the pressure difference.
So if speed is reduced, the pressure in the cylinder should be higher compared to that in higher speed, therefore piston needs to move down further to create a lower pressure (increased volume, lower pressure)before the suction valve can open (the term 'piston moves down further' means the distance traveled by piston have to be more before the suction valve opens, but the overall piston displacement is still the same).

How can I match the flow and speed for the interstage pressure? Any formula to look into? I'm really not good at this.

 

[hysim]

Hi nickelkid,

I just want to see how the valve behave at different pressure and speed, therefore I need to change the speed so that I can change the pressure to record more data.

 

[hysim]

Hi David,

If I speed the machine up, the actual pressure on the top side of the suction valve goes down, so the cylinder pressure required to open the suction valve goes down, wouldn't it be much easier for the valve to open? (i.e the valve opens earlier at higher speed) I once installed an accelerometer to measure the vibration level at the valve cover, it seems that for a 450rpm air compressor, valve opens at 50-60 degree from TDC, while the same compressor moving with 800rpm opens at 30-40 degree.

So if my discharge line is still atmospheric pressure, as speed increases, suction pressure decreases and therefore discharge pressure decreases, but compression ratio remain the same?
Can we know how high is the downstream pressure that will break a rod or fail a mechanical coupling? Any clues to look into it?

Thanks for the inputs in matching the two machines.

 

[LittleInch]

"So if speed is reduced, the pressure in the cylinder should be higher compared to that in higher speed,"

Why? The pressure will be the same or less.

On the up stroke as the gas/air compresses, it will move out of the cylinder once the pressure exceeds the pressure on the outlet. As this is supposed to be the same for both low speed and high speed, then the exhaust valve will open at the same place. the piston continues to the top. As soon as the piston moves back down the exhaust valve will close as soon as there is pressure difference - the same for both situations.

In any dynamic situation not everything will be equal at the different speeds, but close enough o make no real difference IMO. The impact of varying pressure in the inlet is one issue as is the inertia in involved. this analysis only works properly if the outlet pressure stays the same. If it changes with speed then lots of things change.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[nickelkid]

Okay, your constants are suction pressure 1st stage and discharge pressure on the 2nd stage, I think.

If your only variable is speed on the first stage you can establish your interstage pressure at 100% load on the 1st stage and then the resultant pressures as you lower that speed. The 2nd stage needs to be reviewed for rod loading for the change in pressure ratios that result.

 

[zdas04]

Hisym,
I spent all day trying to figure out what you were talking about and finally got it at bed time. You are assuming that the suction valve opens at a specific pressure. It doesn't. It opens at an approximate dP. It it takes 5 psi dP to open the suction valve, then at slow speed at sea level it would open at some valve around 9.7 psia. If I lose 3 psi getting the gas to the suction valve then that valve will open at around 6.7 pai. At a normal discharge pressure of 40 psia (4.1 compression ratios at slow speed, 5.97 compression ratios at high speed) the discharge temperature and rod load will be considerably higher.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist